Golf club and method of designing hollow golf club head

ABSTRACT

The golf club head ( 14 ) of the present invention has a face portion ( 24 ) for striking a golf ball, a crown portion ( 18 ) connected to the face portion ( 24 ), and a sole portion ( 22 ) connected to the face portion ( 24 ). The ratio of either lower of a first equivalent rigidity of a crown member ( 18 ) and a second equivalent rigidity of a sole member ( 22 ) to the higher is equal to or less than 0.75. The present invention provides a golf club ( 10 ) provided with the hollow golf club head ( 14 ) capable of increasing the carry of a golf ball based on a technique for increasing the carry of a golf ball other than conventional ones such as adjustment of the loft angle and reduction in the thickness of the striking surface, and provides a method of designing such a hollow golf club head as well.

TECHNICAL FIELD

The present invention relates to a method of designing a hollow golfclub head which has a face portion for striking a golf ball, a crownportion connected to the face portion, and a sole portion connected tothe face portion. The present invention also relates to a golf clubprovided with such a golf club head.

PRIOR ART

Through the improvement and development of golf club head structures andmaterials, golf club makers at present are proposing various types ofgolf clubs with which even weaker golfers are capable of hitting a golfball a long distance.

The initial ballistic characteristics of a golf ball have been adjustedin some ways. For example, the launch angle of a struck golf ball hasbeen increased by changing the loft angle of a hollow golf club head orthe initial velocity of a golf ball has been increased by reducing thethickness of a golf ball-striking surface to improve the restitutioncharacteristics with respect to a golf ball.

In JP 10-155943 A, a hollow golf club head is disclosed in which a golfball-striking surface has a thin portion formed along itscircumferential edge. Owing to such configuration, elastic deformationof the striking surface is promoted during striking of a golf ball sothat the surface has an increased coefficient of restitution withrespect to the golf ball. Thus, an increase in the carry of a golf ballis achieved.

Further, with a golf club having a golf club head of which the loftangle is made larger within a predetermined range, the launch angle of agolf ball increases and an increase in the carry is thus achieved.

In addition to an increase in the launch angle, the number ofrevolutions (backspin rate) of a golf ball increases when the loft angleis made larger in a golf club having a golf club head of which the loftangle is changed. When the loft angle is made smaller, the launch anglebecomes smaller and the backspin rate of a golf ball also decreases.

Therefore, there is a problem in that, even if a golf club head having alarger loft angle is used in order to increase the carry, the carry doesnot increase very much because the backspin rate increases at the sametime. In other words, since it is a characteristic of the loft anglethat its increase or decrease results in an increase or decrease in thebackspin rate and the launch angle alike, the launch angle cannot beincreased while the backspin rate is decreased, or again, the launchangle cannot be decreased while the backspin rate is increased. That is,there is a problem in that the backspin rate and the launch angle cannotbe changed independently of each other.

Furthermore, even if the characteristics of the loft angle are utilizedwith the intention of providing the optimal golf club for a golfer, agolf club with an inappropriate loft angle may sometimes be providedbecause there exists no guideline for the selection of an appropriategolf club on which every golfer having his or her own golf swing shoulddepend. In such cases, the carry of a golf ball may decrease instead ofincreasing.

In the case where the striking surface of a golf club head is madethinner, on the other hand, the initial velocity of a golf ball can beincreased and the carry can be made longer. However, the mechanicalstrength of the striking surface decreases as a result of the partialreduction in thickness of the surface, thus causing to a problem withdurability.

In order to solve the problems described above, the present inventionhas an object of providing a golf club which has a hollow golf club headcapable of increasing the carry of a golf ball based on a techniqueother than conventional ones such as adjustment of the loft angle andreduction in the thickness of the striking surface. Another object ofthe present invention is to provide a method of designing such a hollowgolf club head.

DISCLOSURE OF THE INVENTION

The above objects are achieved by the present invention providing a golfclub comprising a hollow golf club head which has a face portion forstriking a golf ball, a crown portion connected to the face portion, anda sole portion connected to the face portion, wherein: a first regionwhose surface area constitutes 5% or more of a total surface area of thecrown portion is formed by a first outer shell member in a region of thecrown portion which is located along a connecting edge of the crownportion connecting to the face portion and within a distance of 50 mmfrom the connecting edge, and a second region whose surface areaconstitutes 5% or more of the total surface area of the sole portion isformed by a second outer shell member in a region of the sole portionwhich is located along a connecting edge of the sole portion connectingto the face portion and within a distance of 50 mm from the connectingedge of the sole portion; and when a product of an elastic modulus ofthe first outer shell member in a direction in which a striking surfaceis oriented and a thickness of the first outer shell member in the firstregion is taken as a first equivalent rigidity and a product of anelastic modulus of the second outer shell member in the direction inwhich the striking surface is oriented and a thickness of the secondouter shell member in the second region is taken as a second equivalentrigidity, a ratio of either lower of the first equivalent rigidity andthe second equivalent rigidity to the higher is equal to or less than0.75.

In other words, it is a feature of the present invention that the firstand second regions which allow the ratio as above to be equal to or lessthan 0.75 exist in specified regions of the crown portion and the soleportion of the golf club head, respectively, each of the specifiedregions being located within 50 mm of the connecting edge of therelevant portion to the face portion, and the first and second regionseach have a surface area constituting 5% or more of the total surfacearea of the relevant portion.

Preferably, either or both of the first and second outer shell membersare composed of a composite material in which a fiber reinforced plasticmaterial is laminated. Further, the above ratio is preferably equal toor less than 0.5.

It is preferable that the surface area of the first region constitutes10% or more of the total surface area of the crown portion and thesurface area of the second region constitutes 10% or more of the totalsurface area of the sole portion. It is also preferable that the firstregion exists in a region of the crown portion which is located alongthe connecting edge to the face portion and within 40 mm of theconnecting edge and the second region exists in a region of the soleportion which is located along the connecting edge to the face portionand within 40 mm of the connecting edge.

The present invention also provides a method of designing a hollow golfclub head which has a face portion for striking a golf ball, a crownportion connected to the face portion, and a sole portion connected tothe face portion, wherein: a first region whose surface area constitutes5% or more of a total surface area of the crown portion is formed by afirst outer shell member in a region of the crown portion which islocated along a connecting edge of the crown portion connecting to theface portion and within a distance of 50 mm from the connecting edge; asecond region whose surface area constitutes 5% or more of a totalsurface area of the sole portion is formed by a second outer shellmember in a region of the sole portion which is located along aconnecting edge of the sole portion connecting to the face portion andwithin a distance of 50 mm from the connecting edge of the sole portion;a product of an elastic modulus of the first outer shell member in adirection in which a striking surface is oriented and a thickness of thefirst outer shell member in the first region is taken as a firstequivalent rigidity; and a product of an elastic modulus of the secondouter shell member in the direction in which the striking surface isoriented and the thickness of the second outer shell member in thesecond region is taken as a second equivalent rigidity, the methodcomprising the steps of: holding in advance the characteristic data thatexpresses changes in initial ballistic characteristics of a golf ballcaused when either of the first and second equivalent rigidities ischanged while the other is kept constant; using the held characteristicdata to set a ratio between the first equivalent rigidity and the secondequivalent rigidity in accordance with the initial ballisticcharacteristics of the golf ball struck by a golfer; and employing twomembers which conform to the set ratio as the first and second outershell members.

The ratio between the first equivalent rigidity and the secondequivalent rigidity is to be considered as the ratio of one equivalentrigidity to the other, that is to say, the ratio may be of the firstequivalent rigidity to the second or vice versa.

The characteristic data is prepared for each of plural head speeds atwhich golfers strike golf balls and the above ratio can be set accordingto a head speed. Alternatively, the characteristic data is prepared foreach of loft angles and the above ratio can be set according to a loftangle.

In connection with the method of the present invention, it is preferablethat a composite material in which a fiber reinforced plastic materialis laminated is used for either or both of the first and second outershell members and the above ratio is established by regulating anorientation angle of the composite material.

The present invention further provides a golf club comprising a hollowgolf club head which has a face portion for striking a golf ball, acrown portion connected to the face portion, and a sole portionconnected to the face portion, the golf club being included among aseries of golf clubs adapted for different head speeds, wherein: a firstregion whose surface area constitutes 5% or more of the total surfacearea of the crown portion is formed by a first outer shell member in aregion of the crown portion which is located along a connecting edge ofthe crown portion connecting to the face portion and within a distanceof 50 mm from the connecting edge, and a second region whose surfacearea constitutes 5% or more of the total surface area of the soleportion is formed by a second outer shell member in a region of the soleportion which is located along a connecting edge of the sole portionconnecting to the face portion and within a distance of 50 mm from theconnecting edge of the sole portion; when a product of an elasticmodulus of the first outer shell member in a direction in which astriking surface is oriented and a thickness of the first outer shellmember in the first region is taken as a first equivalent rigidity and aproduct of an elastic modulus of the second outer shell member in thedirection in which the striking surface is oriented and a thickness ofthe second outer shell member in the second region is taken as a secondequivalent rigidity, a ratio of either lower of a first equivalentrigidity and a second equivalent rigidity to the higher is equal to orless than 0.75; and a composite material in which a fiber reinforcedplastic is laminated is used for either or both of the first and secondouter shell members, having an orientation angle of fibers thereofregulated according to the head speed so as to establish the aboveratio.

Finally, the present invention provides a golf club comprising a hollowgolf club head which has a face portion for striking a golf ball, acrown portion connected to the face portion, and a sole portionconnected to the face portion, the golf club being included among aseries of golf clubs with different loft angles, wherein: a first regionwhose surface area constitutes 5% or more of the total surface area ofthe crown portion is formed by a first outer shell member in a region ofthe crown portion which is located along a connecting edge of the crownportion connecting to the face portion and within a distance of 50 mmfrom the connecting edge, and a second region whose surface areaconstitutes 5% or more of the total surface area of the sole portion isformed by a second outer shell member in a region of the sole portionwhich is located along a connecting edge of the sole portion connectingto the face portion and within a distance of 50 mm from the connectingedge of the sole portion; when a product of an elastic modulus of thefirst outer shell member in a direction in which a striking surface isoriented and a thickness of the first outer shell member in the firstregion is taken as a first equivalent rigidity and a product of anelastic modulus of the second outer shell member in the direction inwhich the striking surface is oriented and a thickness of the secondouter shell member in the second region is taken as a second equivalentrigidity, a ratio of either lower of a first equivalent rigidity and asecond equivalent rigidity to the higher is equal to or less than 0.75;and a composite material in which a fiber reinforced plastic islaminated is used for either or both of the first and second outer shellmembers, having an orientation angle of fibers thereof regulatedaccording to a loft angle of the golf club so as to establish the aboveratio.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded perspective view schematically showing a golf clubas an embodiment of the golf club of the present invention; and

FIGS. 2A and 2B are diagrams clearly explaining the deformation causedwhen a golf ball is struck with the golf club.

FIGS. 3A to 3C are diagrams that show changes in the backspin rate of agolf ball with respect to changes in equivalent crown rigidity;

FIGS. 4A to 4C are diagrams that show changes in the launch angle of agolf ball with respect to changes in equivalent crown rigidity; and

FIGS. 5A to 5C are diagrams that show changes in the initial velocity ofa golf ball with respect to changes in equivalent crown rigidity.

FIG. 6 is a diagram explaining the orientation angle in the golf clubhead of the present invention; and

FIGS. 7A and 7B are diagrams explaining the crown portion of the golfclub head of the present invention.

FIG. 8 is a diagram representing changes in the carry of a golf ballaccording to the backspin rate and the launch angle, which are initialballistic characteristics of a golf ball.

FIG. 9 is a schematic diagram showing an exemplary series of golf clubswith different loft angles.

BEST MODE OF IMPLEMENTING THE INVENTION

The golf club of the present invention, and the method of designing ahollow golf club head of the present invention, are described in detailbelow based on preferred embodiments shown in the accompanying drawings.

FIG. 1 is an exploded perspective view schematically showing a golf clubas an embodiment of the golf club of the present invention.

A golf club 10 shown in FIG. 1 is structured so that it has a golf clubshaft 12 provided with a grip portion 13 at one end, and a hollow golfclub head (hereinafter referred to simply as golf club head) 14connected to the other end of the golf club shaft 12.

The golf club shaft 12 is inserted into a neck member 16 and bonded inplace, thus being integrated with the golf club head 14.

The golf club head 14 has a face portion that strikes a golf ball, acrown portion that is connected to the face portion, and a sole portionthat is connected to the face portion, and is provided with a crownmember 18 that forms the major part of the crown portion, a side member20 that mainly forms the side portion, a sole member 22 that forms thesole portion, and a face member 24 that forms the face portion and has agolf ball-striking surface, each as an outer shell member.

The side member 20, the sole member 22, and the face member 24 areintegrated with one another in advance by welding, by using an adhesive,or the like. The side member 20 has an edge which is bent to the crownportion side to provide an extension portion 26 extending in the crownportion to form a part thereof. The face portion 24 has an edge which isbent to the crown portion side to provide an extension portion 28extending in the crown portion to form a part thereof. That is, the sidemember 20, the sole member 22, and the face member 24 are previouslyintegrated with one another into the state as shown in FIG. 1, and thenthe crown member 18 is joined to the extension portions 26 and 28, thegolf club head 14 being thus constructed.

One out of various types of alloy materials is used for the side member20, the face member 24, and the sole member 22, such as a titaniumalloy, an aluminum alloy, or a stainless steel alloy. For the solemember 22, a composite material structured by laminating a fiberreinforced plastic material in a plurality of layers as will bedescribed later and other materials may also be used.

The crown member 18 is structured by a composite material in which aplurality of layers of a carbon fiber reinforced plastic material havingdifferent orientation angles are laminated together. An epoxy resin, anunsaturated polyester resin, a vinyl ester resin, or the like may beused as a matrix. It should be noted that reinforcing fibers other thancarbon fibers, such as glass fibers and aramid fibers, may also be usedin the present invention.

The crown member 18 forms a region whose surface area constitutes 5% ormore of the total surface area of the crown portion (hereafter referredto as the first region) in an area of the crown portion which is locatedalong the connecting edge of the crown portion connecting to the faceportion and within a distance of 50 mm from the connecting edge. Thefirst region formed by the crown member 18 will be described in detaillater.

When an equivalent crown rigidity is defined as the product of theelastic modulus (Young's modulus) of the crown member 18 in thedirection in which the striking surface is oriented and the thickness ofthe crown member 18 and an equivalent sole rigidity is defined as theproduct of the elastic modulus of the sole member 22 in the direction inwhich the striking surface is oriented and the thickness of the solemember 22, the ratio of the equivalent crown rigidity to the equivalentsole rigidity is equal to or less than 0.5 in the embodiment as shown.The elastic modulus in the direction in which the striking surface isoriented is the elastic modulus whose values are obtained in thedirection in which the line of intersection of the crown portion and aplane which is perpendicular to the striking surface of the face portionlies.

The direction in which the striking surface is oriented as referred toabove should be considered as the azimuthal direction which is theoriented direction of the striking surface in a plane parallel to areference plane when the golf club head addressed in an ordinaryposition on the reference plane is looked down on perpendicularly to thereference plane. The expression “addressed in an ordinary position” usedherein means that the golf club head is addressed in accordance with thelie angle, with the central axis of the golf club shaft and the leadingedge of the face portion of the golf club head being found to beparallel to each other. The expression “addressed in accordance with thelie angle” used herein means to be addressed such that the gap betweenthe round surface of the sole portion as the bottom surface of the golfclub head and the reference plane does not essentially vary from the toeside to the heel side. If the round surface of the sole portion isindefinite, the golf club may be addressed such that the scorelines madeon the striking surface are parallel to the reference plane. If theround surface of the sole portion is indefinite and, in addition, thescore lines are hard to determine whether or not to be parallel to thereference plane because of their being not straight lines, and so forth,the lie angle is set using the equation: the lie angle(degrees)=(100−the club length (inches)). In the case of the club lengthof 44 inches, for instance, the lie angle is 100−44=56 degrees.

The club length is measured by the mensuration established by Japan GolfGoods Association. Examples of the measuring instrument to be usedinclude Club Measure II manufactured by Kamoshita Seikoujyo Corporation.

The elastic modulus as above is defined as follows on condition that thegolf club head is addressed in an ordinary position in the horizontalreference plane.

Supposing that there is the plane extending in the direction in whichthe striking surface of the face portion is oriented when the golf clubhead is addressed in an ordinary position in the horizontal referenceplane, that is perpendicular to both the reference plane and thestriking surface, the above elastic modulus is the elastic modulus whosevalues are obtained in the direction in which the line of intersectionof the plane as supposed above and the crown portion lies.

In this embodiment, the backspin rate of the golf ball struck with thestriking surface is reduced and the launch angle of the ball isincreased by setting the ratio as described above to 0.5. According tothe present invention, however, the ratio of the equivalent crownrigidity to the equivalent sole rigidity can be set to 0.75 or less.

FIGS. 2A and 2B are explanatory diagrams for explaining, in an easy tounderstand manner, how a golf ball is struck with the golf club 10.

As shown in FIG. 2A, when a golf ball is struck, an impact force of thegolf ball acts on the striking surface of the face member 24, and theimpact force is transmitted to the crown portion and the sole portion.Now, directing attention to deformations of the crown portion and thesole portion that are generated due to the impact force, the equivalentcrown rigidity is half as high as the equivalent sole rigidity, and thedeformation of the crown portion therefore becomes larger than thedeformation of the sole portion. The striking surface of the face member24 therefore deforms slightly in such a direction as realizing a largerloft angle as shown in FIG. 2B. This deformation of the striking surfacewhen impacted by the golf ball B affects the backspin rate and thelaunch angle of the golf ball B.

FIGS. 3A to 3C show changes in the backspin rate for cases where theequivalent crown rigidity is changed while keeping the equivalent solerigidity (113 GPa·mm) fixed, for head speeds of 34 m/s, 40 m/s, and 46m/s. As shown in FIGS. 3A to 3C, although the amount of change differsaccording to the head speed, it can be understood that the backspin ratedecreases due to the reduction in the equivalent crown rigidity in eachof the cases.

On the other hand, FIGS. 4A to 4C show changes in the launch angle forcases where the equivalent crown rigidity is changed while keeping theequivalent sole rigidity fixed (113 GPa·mm), for head speeds of 34 m/s,40 m/s, and 46 m/s. As shown in FIGS. 4A to 4C, although the amount ofchange differs according to the head speed, it can be understood thatthe launch angle increases due to the reduction in the equivalent crownrigidity in each of the cases.

Further, FIGS. 5A to 5C show changes in the initial velocity of a golfball for cases where the equivalent crown rigidity is changed whilekeeping the equivalent sole rigidity fixed (113 GPa·mm), for head speedsof 34 m/s, 40 m/s, and 46 m/s. As shown in FIGS. 5A to 5C, it can beunderstood that, in each of the cases, there exists a value of theequivalent crown rigidity at which the initial velocity of a golf ballbecomes the maximum.

In order to realize the member whose equivalent crown rigidity is asabove, it is suitable to employ a composite material comprising a fiberreinforced plastic material. The composite material may be so fabricatedas to have 7 or 3 layers and have an equivalent rigidity of a value from0.37 to 5.63 times as large as the reference value as set forth in Table1 below, for instance. In this respect, the reference value is definedas the value of the equivalent rigidity of a five layer compositematerial obtained by laminating 4 layers of carbon fiber reinforcedplastic material, with the orientation angle of them being setalternately to +45 degrees and −45 degrees with respect to thepredetermined reference direction, and piling the uppermost layer ofcarbon fiber reinforced plastic material having an orientation angle of90 degrees onto the laminate formed. The reference direction is definedas the direction in which the line of intersection of the crown portionand the plane perpendicular to the striking surface of the face portionlies.

FIG. 6 illustrates the orientation angle of reinforcing fibers in thecrown member with respect to the direction D in which the strikingsurface is oriented. The fibers having an orientation angle of +45degrees are oriented in the direction as shown by D₁ and those having anorientation angle of −45 degrees in the direction as shown by D₂.

In this invention, the crown member may also be composed of a materialin the form of a fabric-like cross prepreg, in which reinforcing fibersincorporated in fiber reinforced plastic layers are oriented indifferent directions, that is to say, at different orientation anglesof, for instance, −45 degrees and +45 degrees. In that case, a layer ofsuch material formed should be considered to possess a two-layerstructure.

Referring now to Table 1, the member composed of 3 laminated layers eachhaving an orientation angle of 0° or 90°, for instance, is formed suchthat the layers have orientation angles of 90°, 0°, and 90°, from thelowermost to the uppermost layers sequentially. The member composed of 7laminated layers each having an orientation angle of +60°, −60° or 90°is formed such that the layers have orientation angles of +60°, −60°,+60°, −60°, +60°, −60°, and 90°, from the lowermost to the uppermostlayers sequentially.

Graphs shown in FIGS. 3A to 3C, 4A to 4C, and 5A to 5C can be obtainedby manufacturing the golf club head 10 by using such a compositematerial as set forth in the table in the crown member 18, andperforming golf ball striking tests to measure the initial ballisticcharacteristics of a golf ball.

TABLE 1 Equivalent crown rigidity value Number of OrientationOrientation Orientation Orientation laminated layers Thickness angle 0°,90° angle ±30°, 90° angle ±45°, 90° angle ±60°, 90° 3 0.51 mm 2.30 1.260.56 0.37 5 0.85 mm 3.96 2.39 1.00 0.62 7 1.18 mm 5.63 3.52 1.44 0.87

In Table 2 below, values of the equivalent rigidity of various alloymaterials are represented as the ratio to the reference value asdescribed before. The equivalent rigidity of the alloy materials isgenerally high as compared with that of the laminated compositematerials comprising a carbon fiber reinforced plastic material asdescribed above.

TABLE 2 Equivalent crown Material Thickness rigidity value Ti-6-4 alloy1 mm 8.81 SUS 1 mm 15.07 Al alloy 1 mm 5.32 Mg alloy 1 mm 3.37

In the table, Ti-6-4 alloy is the titanium alloy constituted of 6% byweight of Al, 4% by weight of V, and Ti as the remainder. SUS is theprecipitation-hardened stainless steel (stainless steel alloy)constituted of 0.06% by weight of C, 0.4% by weight of Si, 0.6% byweight of Mn, 7.0% by weight of Ni, 17.0% by weight of Cr, 1.2% byweight of Al, and Fe as the remainder.

Al alloy (aluminum alloy) is the alloy constituted of 5.6% by weight ofZn, 2.5% by weight of Mg, 1.6% by weight of Cu, and Al as the remainder.Mg alloy (magnesium alloy) is the alloy constituted of 3.5% by weight ofZn, 0.6% by weight of Zr, and Mg as the remainder.

From these results, it is preferable in order to cause the deformationof the striking surface as shown in FIG. 2B to use an alloy material forthe sole member 22 and a composite material, in which a carbonreinforced fiber plastic material is laminated, for the crown member 18.

The deformation of the striking surface of the face member 24 shown inFIG. 2B can be effectively achieved with such structure by satisfyingcertain conditions. Specifically, the crown member 18 should include thefirst region which exists in the region of the crown portion locatedalong the connecting edge of the crown portion connecting to the faceportion and within 50 mm of the connecting edge and whose surface areaconstitutes 5% or more of the total surface area of the crown portion.Similarly, the sole member 22 should include a second region (hereafterreferred to as the second region) which exists in the region of the soleportion located along the connecting edge of the sole portion connectingto the face portion and within 50 mm of the connecting edge of the soleportion and whose surface area constitutes 5% or more of the totalsurface area of the sole portion.

The first region in the crown member 18 is described in detail withreference to an example of the golf club head as shown in FIG. 7A. Inthis connection, similar definitions are to be given to the secondregion in the sole member 22.

The crown member of the golf club head as shown in FIG. 7A is a singlemember composed of a composite material.

In the case of the golf club head as shown in FIG. 7A, the region of thecrown portion located along the connecting edge of the crown portionconnecting to the face portion and within 50 mm of the connecting edgeis a region R₁ as shown with hatching, which is located along aconnecting edge 19 and within a distance of 50 mm from the connectingedge 19, and the member that is employed in the region R₁ for a regionwhose surface area constitutes 5% or more of the total surface area ofthe crown portion (the first region) is considered as the first regionin the crown member 18 in the present invention.

In the embodiment as described above, the crown member is a singlemember composed of an alloy material, a composite material, or the like.The present invention, however, is not limited to the embodiment and thecrown member may comprise two or more members of different types.

Referring to another example of the golf club head as shown in FIG. 7B,the first region in the crown member comprising two members of differenttypes is described. In the golf club head as shown in FIG. 7B, the crownmember 18 comprises two members composed of two different materials,such as an alloy material and a composite material, respectively.

In a region R₂, two different members are arranged in a layered manner(for instance, as the lower layer of a titanium alloy and the upperlayer of a fiber reinforced plastic material laminated in itself in 5layers), while a single member (for instance, a layer of a fiberreinforced plastic material laminated in itself in 5 layers) is used ina region R₃. In this situation, the member that is employed for a partof the region R₂ overlapping the region of the crown portion locatedalong the connecting edge 19 of the crown portion connecting to the faceportion and within 50 mm of the connecting edge (see the region R₁ inFIG. 7A) is considered as the first region in the crown member 18 in thepresent invention as long as the surface area of the part constitutes 5%or more of the total surface area of the crown portion. The member thatis employed for a part of the region R₃ overlapping the region R₁ isalso considered as the first region in the crown member 18 in thepresent invention as long as the surface area of the part constitutes 5%or more of the total surface area of the crown portion.

Thus, in the case of such a golf club head as shown in FIG. 7B, aplurality of first regions may be defined in the crown member 18. Itshould be noted that a ratio of the equivalent crown rigidity to theequivalent sole rigidity equal to or less than 0.75 has only to beattained with one of a plurality of first regions. Similar to the firstregion, a plurality of second regions may be defined in the sole member22.

As described above, the deformation of the striking surface of the facemember 24 as shown in FIG. 2B is effectively achieved in the golf clubhead of the golf club according to the present invention. It is afeature of the present invention that the first and second regions withwhich the ratio of the equivalent crown rigidity to the equivalent solerigidity stands at 0.75 or less, preferably at 0.5 or less, lies in thecrown portion and the sole portion, respectively, with the first regionin the crown portion, as having a surface area constituting 5% or moreof the total surface area of the crown portion, being in a region of thecrown portion located within 50 mm of the connecting edge of the crownportion connecting to the face portion, and the second region in thesole portion, as having a surface area constituting 5% or more of thetotal surface area of the sole portion, being in a region of the soleportion located within 50 mm of the connecting edge of the sole portionconnecting to the face portion. There are no particular limitationsplaced on the positions of the first and second regions, provided thatthe regions each lies in a region located along the connecting edge ofthe relevant portion connecting to the face portion and within 50 mm ofthe connecting edge. It is preferable, however, that the first andsecond regions each lie in a region located within 40 mm of theconnecting edge to the face portion. Further, it is preferable that thefirst and second regions each has a surface area constituting 10% ormore of the total surface area of the relevant portion. In this respect,the first and second regions may each be formed by an outer shell membercomposed of a single alloy material or that composed of a laminatedcomposite material. There are of course no limitations placed on thethickness of the outer shell members in the first and second regions,provided that the members allow the ratio as referred to above to be0.75 or less, preferably 0.5 or less.

The total surface area of the crown portion is the total surface area ofa zone enclosed by the edges of the crown portion connecting to the sideportion, the face portion and the neck member 16, respectively, and suchconnecting edges can be found out based on the change in radius ofcurvature on the periphery of the crown portion. Similarly, the totalsurface area of the sole portion is the total surface area of a zoneenclosed by the edges of the sole portion connecting to the side portionand the face portion, respectively. If the crown portion is indefinitedue to the painting on the outer surface of a golf club head, the golfclub head may be decomposed so as to inspect joining parts from insideand thereby find out the edges of the side portion, the crown portion,and the sole portion. In the case of the crown portion still beingindefinite, it is also possible to consider the projected area of thegolf club head except for the striking surface, that is found by lookingdown on a golf club head perpendicularly to the plane on which the golfclub head is placed such that the striking surface is oriented properlyin line with its loft angle, as the total surface area of the crownportion.

If the crown member 18 or the sole member 22 has certain thicknessdistribution, the average thickness of the member is considered as itsthickness. In the golf club head of the golf club according to thepresent invention, as described before, the first and second regionswhich allow the ratio of the equivalent crown rigidity to the equivalentsole rigidity to be 0.75 or less, preferably 0.5 or less, lie in theregions of the crown portion and the sole portion, each located within50 mm of the connecting edge of the relevant portion connecting to theface portion, and they each have a surface area constituting 5% or moreof the total surface area of the relevant portion. It is also the casewith the outer shell members in the first and second regions as such, sothat the average thickness of the outer shell member having a thicknessdistribution, if any, is considered as its thickness.

With the golf club 10, the backspin rate of the struck golf ball can bereduced and its launch angle can be increased by the deformation of thestriking surface when impacted as shown in FIG. 2B because of the ratioof the equivalent crown rigidity to the equivalent sole rigidity of thegolf club head 14 being equal to or less than 0.5, as described above.

It should be noted that, although a structure is used in the embodimentdescribed above where the ratio of the equivalent crown rigidity to theequivalent sole rigidity is equal to or less than 0.5, it is alsopossible according to the present invention to use a structure in whichthe ratio of the equivalent sole rigidity to the equivalent crownrigidity is equal to or less than 0.75, preferably equal to or less than0.5. In other words, the ratio of the equivalent crown rigidity to theequivalent sole rigidity may also be set to 4/3 or more, preferably to 2or more.

In that case, the initial ballistic characteristics of a golf ball maybe adjusted on the golf club head side such that the backspin rate beincreased and the launch angle reduced. Specifically, a compositematerial in which a carbon fiber reinforced plastic material islaminated in layers may be used for the sole member 22 and any ofvarious alloy materials including titanium alloys, aluminum alloys, andstainless steel alloys may be used for the crown member 18. The solemember 22 composed of a composite material will be joined integrallywith the bonding surfaces provided on the side member 20 and the facemember 24 using an adhesive and so forth. This type of golf club headreadily allows a golf ball to follow a trajectory at a lower level sothat it is most suitable for golfing on a windy day.

Further, a composite material in which a plurality of layers of a fiberreinforced plastic material are laminated may be used for both the crownmember 18 and the sole member 22 at a time. In this respect, there isnothing required but that the ratio of either lower of the equivalentcrown rigidity and the equivalent sole rigidity to the higher be equalto or less than 0.75.

Thus according to the present invention, the backspin rate and thelaunch angle can be adjusted separately from each other, whereas thesetwo characteristics should be increased or decreased alike when aconventional change in the loft angle of a golf club head is performed.

A method for designing the golf club as above is described withreference to FIG. 8. FIG. 8 is a chart representing changes in the carryof a golf ball according to the backspin rate and the launch angle, eachas an initial ballistic characteristic of a golf ball. The chart showsthe relationship between the backspin rate and the launch angle withcontours, based on their values bringing about the same carry of a golfball at a fixed head speed (head speed of 40 m/sec). As an example, whena golfer strikes a golf ball at a head speed of 40 m/sec, with theinitial ballistic characteristics of the golf ball being such that thebackspin rate is equal to 2,800 and the launch angle is equal to 12degrees, the carry of the golf ball is nearly 236 yards.

In this case, in order to effectively increase the carry, the golfermust shift the backspin rate and the launch angle not in direction B butdirection A shown in FIG. 8, i.e., a direction in which the launch angleincreases and the backspin rate decreases. Such shift in direction Acannot be achieved by conventional adjustments of the loft angle becausethe launch angle and the backspin rate are increased or decreased alike.However, the shift in direction A can be achieved by using a structurein which the ratio of the equivalent crown rigidity to the equivalentsole rigidity is equal to or less than 0.75, preferably equal to or lessthan 0.5, as described above.

By knowing the initial ballistic characteristics of the golf ball struckby a golfer (the initial velocity, backspin rate, and launch angle), adirection for the increase in the carry of the golf ball can be foundout in the chart shown in FIG. 8. It is then preferable to set thebackspin rate and the launch angle so that they may be shifted in thedirection thus found out and determine the type of the materials for thecrown member 18 and the sole member 22 (type of alloy and type of fiberreinforced plastic material) and the member structure (orientation anglein a laminated material, for instance) so that they may conform to thedirection, that is to say, may allow the ratio of the equivalent crownrigidity to the equivalent sole rigidity to be equal to or less than0.75.

To be more specific, the golf club head can be designed as follows: Suchcharacteristic data as shown in FIGS. 3A to 3C, FIGS. 4A to 4C, or FIGS.5A to 5C is held in advance, which expresses the initial ballisticcharacteristics of a golf ball by using either or both of the equivalentcrown rigidity in the first region formed by the crown member 18 and theequivalent sole rigidity in the second region formed by the sole member22 as a parameter, with the first region being in a region of the crownportion which is located along the connecting edge of the crown portionconnecting to the face portion and within a distance of 50 mm from theconnecting edge and whose surface area constitutes 5% or more of thetotal surface area of the crown portion, and the second region being ina region of the sole portion which is located along the connecting edgeof the sole portion connecting to the face portion and within a distanceof 50 mm from the connecting edge of the sole portion and whose surfacearea constitutes 5% or more of the total surface area of the soleportion. A direction desirable for the increase in the carry, such asdirection A in FIG. 8, is found out based on the initial ballisticcharacteristics of the golf ball struck by a golfer and the ratiobetween the equivalent crown rigidity and the equivalent sole rigidityis set using the held characteristic data so that the shift in thefound-out direction may be effected. The members that conform to the setratio are employed as the outer shell member whose surface areaconstitutes 5% or more of the total surface area of the crown portionand the outer shell member whose surface area constitutes 5% or more ofthe total surface area of the sole portion and arranged in the regionsof the crown portion and the sole portion, each located along theconnecting edge of the relevant portion connecting to the face portionand within 50 mm of the connecting edge, respectively.

In the embodiment as described above, the ratio of the equivalent crownrigidity to the equivalent sole rigidity is set to 0.5 or less so as toadjust the initial ballistic characteristics of a golf ball in thedirection allowing the increase in the launch angle and the decrease inthe backspin rate (direction A) as shown in FIG. 8.

The initial ballistic characteristics of a golf ball, however, may alsobe adjusted such that the backspin rate is increased while the launchangle is decreased. In that case, the ratio of the equivalent solerigidity to the equivalent crown rigidity may be set to 0.75 or less,preferably 0.5 or less.

In other words, a golf club head with which the backspin rate and thelaunch angle are changed independently of each other can be designed bysetting the ratio of either lower of the equivalent crown rigidity andthe equivalent sole rigidity to the higher to 0.75 or less.

The above designing method can be followed by computer.

In this respect, the characteristic data differs according to the headspeed, as shown in FIGS. 3A to 3C, FIGS. 4A to 4C, and FIGS. 5A to 5C.Therefore, in order to quantitatively determine the ratio between theequivalent crown rigidity and the equivalent sole rigidity to therebyensure an increase in the carry, it is preferable to set the ratiobetween the equivalent crown rigidity and the equivalent sole rigidityaccording to the head speed.

Moreover, the golf club provided with such a hollow golf club head asdescribed above can be brought to market as one in a series of golfclubs in which the orientation angle of a composite material such as afiber reinforced plastic material for the crown member or the solemember is determined differently according to the head speed. The headspeed at which individual golfers strike a golf ball may be typified in43 m/sec, 40 m/sec, and 37 m/sec, for instance. Under such conditions,the orientation angle of a composite material in the golf club head isset to ±30° for a golf club adapted for a head speed of 43 m/sec, ±45°for a golf club adapted for a head speed of 40 m/sec, and ±60° for agolf club adapted for a head speed of 37 m/sec. It should be noted thatthe rigidity of a composite material in the golf club head is decreasedas the orientation angle of the material is increased in magnitude from±30° to ±45°, then to ±60°.

In this way, a series of golf clubs classified by different head speedsat which golfers respectively strike a golf ball can be brought tomarket. The term “a series of golf clubs” herein used means a group ofgolf clubs which are designed for a certain distinctive structure andperformance under one and the same brand name, model name, trade name,type designation, variation designation, and so forth. In addition to agroup of golf clubs having the distinctive structure and performanceattained therein, a group of golf clubs which are indicated to bedesigned for the distinctive structure and performance in theinstructions for golf clubs or through a catalogue, a poster or paneldisplayed at the store, a TV commercial message, a promotive video,which are published or produced by a golf club maker or a golf clubsales company, various advertising medias exemplified by atelecommunication or the like, are also considered to have thedistinctive structure or performance attained therein and included inthe present invention as “a series of golf clubs”.

By using the method of designing a hollow golf club head as above, acustom-made golf club head can be provided by setting the ratio betweenthe equivalent crown rigidity and the equivalent sole rigidity inaccordance with the initial ballistic characteristics of the golf ballstruck by a specified golfer. Golf clubs that are designed by settingthe ratio between the equivalent crown rigidity and the equivalent solerigidity in accordance with such initial ballistic characteristics of agolf ball as presumed can also be brought to market.

FIG. 9 is a schematic illustration of an exemplary series of golf clubswith different loft angles of the present invention, showing three golfclubs different from one another in loft angle.

Each golf club has the golf club shaft 12 provided with the grip portion13 at one end and the golf club head 14 as described above at the other.The golf club head 14 is attached to the golf club shaft 12 via a socketof a hosel projected upwardly from the head on the heel side. In thegolf club heads 14 a to 14 c of the respective golf clubs, theequivalent rigidity ratio is established by differently determining theorientation angle of a composite material in which a fiber reinforcedplastic is laminated.

The equivalent rigidity ratio as referred to above is the ratio ofeither lower of the equivalent crown rigidity and the equivalent solerigidity to the higher and is obtained by adjusting either or both ofthe equivalent crown rigidity and the equivalent sole rigidity.

As seen from Table 1, the equivalent rigidity is increased as theorientation angle of a composite material is changed from ±60° and 90°to ±45° and 90°, then to ±30° and 90°, and then to 0° and 90°.Accordingly, a desired equivalent rigidity ratio can be established byregulating the orientation angle of a composite material.

A method of designing such golf clubs as shown is now described. Byknowing the initial ballistic characteristics of the golf ball struck bya golfer (the initial velocity, backspin rate, and launch angle),desirable values of the backspin rate and the launch angle whichincrease the carry of the golf ball are found out in the chart of FIG.8. The backspin rate and the launch angle are set to the values thusfound out independently of each other and the type of the materials forthe crown member and the sole member (type of fiber reinforced plasticmaterial) and the member structure (orientation angle in a laminatedmaterial) are so determined as to conform to them.

The golf club head of each golf club is designed, for instance, asfollows: Characteristic data which expresses the initial ballisticcharacteristics of a golf ball by using either or both of the equivalentcrown rigidity and the equivalent sole rigidity as a parameter (forinstance, data showing changes in the backspin rate or launch angle of agolf ball with respect to the changes in the equivalent crown rigidityor the equivalent sole rigidity) is held in advance according to theloft angle.

Desirable values of the backspin rate and the launch angle are found outin the chart of FIG. 8 based on the initial ballistic characteristics ofthe golf ball struck by a golfer and then, the ratio between theequivalent crown rigidity and the equivalent sole rigidity is set usingthe characteristic data held according to the loft angle so that thebackspin rate and the launch angle are shifted toward the found-outdesirable values. The members each having an orientation angleconforming to the set ratio are employed as the outer shell member whosesurface area constitutes 5% or more of the total surface area of thecrown portion and the outer shell member whose surface area constitutes5% or more of the total surface area of the sole portion and arranged inthe regions of the crown portion and the sole portion, each locatedalong the connecting edge of the relevant portion connecting to the faceportion and within 50 mm of the connecting edge, respectively.

This designing method can be followed by computer.

Since the characteristic data differs according to the loft angle, theratio between the equivalent crown rigidity and the equivalent solerigidity is set for each loft angle value selected for design so as toquantitatively determine the ratio between the equivalent crown rigidityand the equivalent sole rigidity to thereby ensure an increase in thecarry.

The backspin rate and the launch angle can be adjusted more freely andin a more dynamic manner especially because of their adjustments beingcarried out by changing not only the equivalent rigidity ratio but alsothe loft angle.

It is seen from Table 1 that the equivalent crown rigidity is increasedas the number of laminated layers becomes larger, which indicates thatthe equivalent rigidity of a member composed of a composite material canbe adjusted by regulating the number of laminated layers. Inconsequence, the equivalent rigidity ratio can be established byregulating the number of laminated layers of a composite material apartfrom regulating the orientation angle thereof.

The golf club provided with such a hollow golf club head as above can bebrought to market as one in a series of golf clubs in which theorientation angle of a composite material such as a fiber reinforcedplastic material for the crown member or the sole member is determineddifferently according to the loft angle.

The brand name, model name, trade name, type designation, variationdesignation, or the like of the series of golf clubs may be presentedthrough advertising media and then a golfer who is going to purchase agolf club can select the golf club provided with the golf club head witha particular loft angle by referring to the trade name or the typedesignation so as to obtain the initial ballistic characteristics of agolf ball as desired.

EXAMPLE A

The carry of a golf ball was measured using the golf club of the presentinvention to examine the effects with respect to the ratio of theequivalent crown rigidity to the equivalent sole rigidity.

Various golf clubs (Examples 1 to 5 and Comparative Examples 1 and 2)were fabricated by using the hollow golf club head shown in FIG. 1 asthe hollow golf club head of the present invention. The ratio of theequivalent crown rigidity to the equivalent sole rigidity was differedfrom head to head by changing the equivalent crown rigidity as shown inTable 3 below.

For the crown member 18, the composite material which comprises aplurality of laminated layers of a carbon fiber reinforced plasticmaterial containing an epoxy resin as the matrix and carbon fibers withan elastic modulus of 24×10³ (kgf/mm²) as reinforcing fibers, with theorientation angle of the carbon fibers being alternated layer by layer,was used. The 6-4 titanium alloy shown in Table 2 was used for the sidemember 20, the sole member 22, and the face member 24.

TABLE 3 Comparative Comparative Example 1 Example 2 Example 3 Example 4Example 5 Example 1 Example 2 Equivalent crown rigidity (GPa · mm) 12.545.2 55.4 72.3 83.6 90.4 113 Equivalent sole rigidity (GPa · mm) 113 113113 113 113 113 113 Equivalent crown rigidity/ 0.11 0.40 0.49 0.64 0.740.80 1.00 Equivalent sole rigidity Average carry (index number) 140 140138 122 120 102 100

The carry measurement was conducted by performing the golf ball-strikingtest by five golfers as testers on the fabricated golf clubs five timesrepeatedly so as to get the average carry for each club. Average carrieswere represented by index numbers based on the average carry ofComparative Example 2 as the reference (index number of 100), with alarger one being represented by a larger number.

The index numbers of the average carries are set forth in Table 3 as themeasurement results. It can be seen from the measurement results thatthe average carry increased greatly when the ratio of the equivalentcrown rigidity to the equivalent sole rigidity was equal to or less than0.75 (Example 5 as compared with Comparative Example 1), and that theaverage carry increased even further when the ratio was equal to or lessthan 0.5 (Example 3 as compared with Example 4).

EXAMPLE B

The carry of a golf ball was again measured using the golf club of thepresent invention to examine the effects with respect to the ratio (%)of the surface area of the first region of the crown portion to thetotal surface area of the crown portion, with the first region allowingthe ratio of the equivalent crown rigidity to the equivalent solerigidity of 0.5 or less (of 0.4). Specifically, such examination wasconducted by variously changing the first region of the crown portionallowing the ratio of the equivalent crown rigidity to the equivalentsole rigidity of 0.4 in surface area, thus causing the above ratio ofits surface area to vary. The first region was in a region of the crownportion located within 50 mm of the connecting edge of the portionconnecting to the face portion.

The member used in the first region of the crown portion was composed ofa carbon fiber reinforced plastic material containing an epoxy resin asthe matrix and carbon fibers with an elastic modulus of 24×10³ (kgf/mm²)as reinforcing fibers and had an equivalent crown rigidity of 45.2(GPa·mm). The 6-4 titanium alloy shown in Table 2 was used for themembers in the sole portion, the face portion, and the side portion, aswell as the crown portion other than the first region. The equivalentsole rigidity was 113 (GPa·mm).

As shown in Table 4 below, the ratio of the surface area of the firstregion to the total surface area of the crown portion was changed from 3to 70% (Examples 6 to 10 and Comparative Examples 3 and 4), and changesin the carry were examined.

TABLE 4 Comparative Comparative Example 6 Example 7 Example 8 Example 9Example 10 Example 3 Example 4 Ratio of surface area (%) 70 50 30 10 5 43 Average carry (index number) 140 130 125 120 112 101 100

The carry measurement was conducted by performing the golf ball-strikingtest by five golfers as testers on the fabricated golf clubs five timesrepeatedly so as to get the average carry for each club. Average carrieswere represented by index numbers based on the average carry ofComparative Example 2 as the reference (index number of 100), with alarger one being represented by a larger number.

The index numbers of the average carries are set forth in Table 4 as themeasurement results. It can be seen from the measurement results thatthe increase in the average carry was small when the surface area ratiowas equal to or less than 4% (Comparative Examples 3 and 4), while theaverage carry increased greatly when the surface area ratio was equal toor greater than 5%, with a value of 5% as the threshold. In particular,it can be seen that the average carry increased even further when thesurface area ratio was equal to or greater than 10%.

Effects of the present invention become apparent from Examples A and Bdescribed above.

The golf club of the present invention and the method of designing ahollow golf club head of the present invention are described in detailabove. However, the present invention is not limited to the embodimentsdescribed above. Various types of improvements and modifications may ofcourse be made without departing from the gist of the present invention.

INDUSTRIAL APPLICABILITY

As described in detail above, the present invention can increase thecarry of a golf ball based on a technique other than conventional onessuch as adjustment of the loft angle and reduction in the thickness ofthe striking surface because, according to the present invention, theratio of either lower of the equivalent crown rigidity and theequivalent sole rigidity to the higher is equal to or less than 0.75 sothat it is possible to, for instance, decrease the backspin rate andincrease the launch angle. The present invention also makes it possibleto design a golf club head possessing such features as above. Inaddition, the present invention can provide a series of golf clubsadapted for different head speeds, or a series of golf clubs withdifferent loft angles, in which the orientation angle is determineddifferently according to the loft angle.

1. A golf club comprising a hollow golf club head which has a faceportion for striking a golf ball, a crown portion connected to the faceportion, a sole portion connected to the face portion and a side portionconnected to the face portion, the crown portion and the sole portion,wherein: a first region whose surface area constitutes 5% or more of atotal surface area of the crown portion is formed by a first outer shellmember in a region of the crown portion which is located along aconnecting edge of the crown portion connecting to the face portion andwithin a distance of 50 mm from the connecting edge, and a second regionwhose surface area constitutes 5% or more of the total surface area ofthe sole portion is formed by a second outer shell member in a region ofthe sole portion which is located along a connecting edge of the soleportion connecting to the face portion and within a distance of 50 mmfrom the connecting edge of the sole portion, the first outer shellmember structured by a carbon fiber reinforced plastic material and thesecond outer shell member structured by a metal alloy; and when anproduct of an elastic modulus of the first outer shell member in adirection in which a striking surface is oriented and a thickness of thefirst outer shell member in the first region is taken as a firstequivalent rigidity and a product of an elastic modulus of the secondouter shell member in the direction in which the striking surface isoriented and a thickness of the second outer shell member in the secondregion is taken as a second equivalent rigidity, a ratio of either thelower of the first equivalent rigidity and the second equivalentrigidity to the higher is equal to or less than 0.75, wherein a sidemember of the side portion has an edge which is bent to a side of thecrown portion to provide an extension portion extending in the crownportion to form a part thereof, wherein a face member of the faceportion has an edge which is bent to a side of the crown portion toprovide an extension portion extending in the crown portion to form apart thereof, and wherein the first outer shell member is joined to theextension portions from an outside of a hollow region formed by the faceportion, the crown portion, the side portion and the sole portion. 2.The golf club according to claim 1, wherein: the face portion isstructured by a metal alloy.
 3. A method of designing a hollow golf clubhead which has a face portion for striking a golf ball, a crown portionconnected to the face portion, a sole portion connected to the faceportion and a side portion connected to the face portion, the crownportion and the sole portion, wherein: a first region whose surface areaconstitutes 5% or more of the total surface area of the crown portion isformed by a first outer shell member in a region of the crown portionwhich is located along a connecting edge of the crown portion connectingto the face portion and within a distance of 50 mm from the connectingedge; a second region whose surface area constitutes 5% or more of thetotal surface area of the sole portion is formed by a second outer shellmember in a region of the sole portion which is located along aconnecting edge of the sole portion connecting to the face portion andwithin a distance of 50 mm from the connecting edge of the sole portion;a product of an elastic modulus of the first outer shell member in adirection in which a striking surface is oriented and a thickness of thefirst outer shell member in the first region is taken as a firstequivalent rigidity; and a product of an elastic modulus of the secondouter shell member in the direction in which the striking surface isoriented and a thickness of the second outer shell member in the secondregion is taken as a second equivalent rigidity, the method comprisingthe steps of: holding in advance the characteristic data that expresseschanges in initial ballistic characteristics of a golf ball caused wheneither of the first and second equivalent rigidities is changed whilethe other is kept constant; using the held characteristic data to set aratio between the first equivalent rigidity and the second equivalentrigidity in accordance with the initial ballistic characteristics of thegolf ball struck by a golfer; and employing two members that conform tothe set ratio as the first and second outer shell members, wherein aside member of the side portion has an edge which is bent to a side ofthe crown portion to provide an extension portion extending in the crownportion to form a part thereof; and wherein a face member of the faceportion has an edge which is bent to a side of the crown portion toprovide an extension portion extending in the crown portion to form apart thereof.
 4. The method of designing a hollow golf club headaccording to claim 3, wherein: said characteristic data represents eachof plural head speeds at which golfers strike golf balls; saidcharacteristic data is prepared for each of plural head speeds at whichgolfers strike golf balls; and said ratio is set according to a headspeed at which a golfer strikes golf balls.
 5. The method of designing ahollow golf club head according to claim 4, wherein: a compositematerial in which a fiber reinforced plastic material is laminated isused for said first outer shell member and a metal alloy for said secondouter shell member; and said ratio is established by regulating anorientation angle of the composite material.
 6. The method of designinga hollow golf club head according to claim 3, wherein: saidcharacteristic data represents each of plural loft angles of golf clubs;said characteristic data is prepared for each of plural loft angles; andsaid ratio is set according to a loft angle of the golf clubs.
 7. Themethod of designing a hollow golf club head according to claim 3,wherein: the first outer shell member is joined to the extensionportions from an outside of a hollow region formed by the face portion,the crown portion, the side portion and the sole portion.
 8. A golf clubcomprising a hollow golf club head which has a face portion for strikinga golf ball, a crown portion connected to the face portion, a soleportion connected to the face portion and a side portion connected tothe face portion, the crown portion and the sole portion, wherein: afirst region whose surface area constitutes 5% or more of a totalsurface area of the crown portion is formed by a first outer shellmember in a region of the crown portion which is located along aconnecting edge of the crown portion connecting to the face portion andwithin a distance of 50 mm from the connecting edge, and a second regionwhose surface area constitutes 5% or more of the total surface area ofthe sole portion is formed by a second outer shell member in a region ofthe sole portion which is located along a connecting edge of the soleportion connecting to the face portion and within a distance of 50 mmfrom the connecting edge of the sole portion, the face portion beingstructured by a metal alloy; and when a product of an elastic modulus ofthe first outer shell member in a direction in which a striking surfaceis oriented and a thickness of the first outer shell member in the firstregion is taken as a first equivalent rigidity and a product of anelastic modulus of the second outer shell member in the direction inwhich the striking surface is oriented and a thickness of the secondouter shell member in the second region is taken as a second equivalentrigidity, a ratio of either the lower of the first equivalent rigidityand the second equivalent rigidity to the higher is equal to or lessthan 0.75, wherein a side member of the side portion has an edge whichis bent to a side of the crown portion to provide an extension portionextending in the crown portion to form a part thereof, wherein a facemember of the face portion has an edge which is bent to a side of thecrown portion to provide an extension portion extending in the crownportion to form a part thereof, and wherein the first outer shell memberis joined to the extension portions from an outside of a hollow regionformed by the face portion, the crown portion, the side portion and thesole portion.
 9. The golf club according to claim 8, wherein: the golfclub is included among a series of golf clubs adapted for different headspeeds; and a composite material in which a fiber reinforced plastic islaminated is used for either or both of the first and second outer shellmembers, having an orientation angle of fibers thereof regulatedaccording to a head speed so as to establish said ratio.
 10. The golfclub according to claim 8, wherein: the golf club is included among aseries of golf clubs with different loft angles; and a compositematerial in which a fiber reinforced plastic is laminated is used foreither or both of the first and second outer shell members, having anorientation angle of fibers thereof regulated according to a loft angleof the golf club so as to establish said ratio.